Method used to build prompts under the hood does not allow for full format customization

Currently method _build_prompt is implemented in very opinionated way regarding formatting how to display choices or default value. Considering how good of a job the rest of the features do in regards to potential extensions and customizations, I am surprised that the best way I found to customize this method is to overwrite it before use, for example as follows:

import click

def my_own_build_prompts(*args, **kwargs):
    pass

click.termui._build_prompt = my_own_build_prompts

Even though it is "technically" possible to override, I'd argue the fact that in order to find it I had to dig deep into codebase and find the details of implementation feels "hacky" and "not intended" - and for sure error prone and not future-proof.

I am very happy to contribute and implement the solution, but I'd rather hear some opinions from the library owners beforehand whether this is something they'd find valuable.

Start by reading _build_prompt in src/click/termui.py at the linked lines and review the issue discussion. Before changing code, get maintainer agreement on the intended customization API and its formatting scope. Done means an accepted, documented extension path replaces the current need to overwrite click.termui._build_prompt.